Water absorbing polymer

ABSTRACT

A process for producing a substantially dry polymer particle powder. A mixture of polymerization reagents is formed from a mixture of at least one monomer source and a solvent selected from the group consisting essentially of water and organic solvents and an initiator source. The mixture of polymerization reagents is sprayed into a heated, controlled atmosphere, forming droplets of the mixture which are allowed to fall through the heated, controlled atmosphere for a sufficient period of time to obtain a desired degree of polymerization. The solvent is continuously evacuated from the atmosphere during the polymerization process.

This application is a continuation application of U.S. Ser. No.08/973,574, filed Mar. 9, 1998 now U.S. Pat. No. 6,291,605 which is anational stage application under 35 U.S.C. Sec. 371 of Freeman et al.PCT/US96/09132 entitled “A Polymerization Process, Apparatus andPolymer”, filed Jun. 6, 1996, which is a continuation-in-partapplication of U.S. Ser. No. 08/486,079, entitled “A Polymer Produced ByA Radical Polymerization Process”, filed Jun. 7, 1995, now abandoned,which is a continuation-in-part of U.S. Ser. No. 07/855,458, entitled“Method and Apparatus of Continuous Production of Water-AbsorbingPolymeric Spheres and the Product Thereof”, filed Mar. 19, 1992, nowabandoned, which is a continuation of U.S. Ser. No. 07/534,177, entitled“Method and Apparatus for Continuous Production of Water-AbsorbingPolymeric Spheres and the Product Thereof”, filed Jun. 6, 1990, nowabandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a spray polymerization process, anapparatus for producing a dry polymer and a polymer having novelphysical characteristics. Particularly, the present invention relates toa polymerization process for the continuous production in a controlledatmosphere of a substantially dry polymer particle powder comprisingpolymer particles of desired size, shape and density from a liquidmonomer source.

It is known in the art that polymers may be synthesized by steppolymerization and chain polymerization processes. Chain polymerizationis initiated by a reactive species produced by a compound or compoundsreferred to as an initiator. Generally, monomers show varying degrees ofselectivity with regard to the type of reactive center that will causechain polymerization. Monomers show high selectivity between anionic andcationic initiators, however, most monomers will undergo polymerizationwith a radical initiator, although at varying rates. Examples of thetypes of monomers which will polymerize to high molecular weightpolymers in the presence of a radical initiator include: ethylene;1,3-dienes; styrene; halogenated olefins; vinyl esters; acrylates;methacrylates; acrylonitrile; methacrylonitrile; acrylamide;methacrylamide; N-vinyl carbazole; N-vinyl pyrrolidone.

Essentially, radical polymerization conditions are either homogenous orheterogeneous, depending upon whether the initial reaction mixture ishomogenous or heterogeneous. Some homogeneous systems however, maybecome heterogeneous as polymerization proceeds due to the insolubilityof the polymer in the reaction media. Generally, mass and solutionpolymerizations are homogeneous processes, while suspension and emulsionpolymerizations are heterogeneous processes. All monomers can bepolymerized by any of the various processes however, it is usually foundthat for commercial considerations the polymerization of a particularmonomer is best carried out by one or two of the processes.

Bulk or mass polymerization of a pure monomer offers the simplestprocess with a minimum of contamination of the product. Bulkpolymerization, however, is difficult to control due to thecharacteristics of radical chain polymerization. The bulk process ishighly exothermic, high activation energies are involved, and there is atendency toward the gel effect. Such characteristics make thedissipation of heat difficult, therefor, careful temperature control isrequired during bulk polymerization processes. Additionally, theviscosity of the reaction system increases rapidly at a relatively lowconversion, thereby requiring the use of elaborate stirring equipment.Localized “hot spots” may occur which damage, degrade and discolor thepolymer product, and a broadened molecular weight distribution mayresult due to chain transfer between polymer molecules. There is alsothe risk in extreme cases that an uncontrolled acceleration of thepolymerization rate can lead to disastrous runaway-type reactions.

Many of the disadvantages of bulk polymerization may be overcome bypolymerizing a monomer in a solvent (solution polymerization). Thesolvent, which may be water, acts as a diluent and aids in the transferof the heat of polymerization. The solvent can be easily stirred sincethe viscosity of the reaction mixture is decreased. Although thermalcontrol of a solution polymerization process is easier than with mass orbulk polymerization, the purity of the polymer may be affected if thereare difficulties in removing the solvent during and followingpolymerization.

Heterogeneous polymerization is used extensively to control the thermalviscosity problems often associated with homogeneous processes.Precipitation polymerization is a heterogeneous polymerization processwhich begins as a homogeneous polymerization but converts toheterogeneous polymerization. A monomer either in bulk or in solution(usually aqueous but sometimes organic) forms an insoluble polymer inthe reaction medium. Precipitation polymerization can be referred to aspowder or granular polymerizations because of the forms in which thefinal polymer products are obtained. The initiators used inprecipitation polymerization are soluble in the initial reaction mediumand polymerization proceeds following absorption of monomer into thepolymer particles.

Suspension polymerization, also referred to as bead or pearlpolymerization, is carried out by suspending the monomer (discontinuousphase) as droplets (50 to 500 μm in diameter) in water (continuousphase). The ratio of water to monomer typically will vary from about 1:1to 4:1 in most polymerizations. The monomer droplets which aresubsequently converted to polymer particles do not coalesce due toagitation and the presence of suspension stabilizers also referred to asdispersants or surfactants. Stabilizers may be water soluble polymers orwater insoluble inorganic powders. The suspension stabilizers are usedtypically in an amount that is less than 0.1 weight percent of theaqueous phase. The two-phase suspension system cannot be maintained insuspension polymerization without agitation.

Suspension polymerization initiators are soluble in the monomer dropletsand are referred to as oil-soluble initiators. Suspension polymerizationin the presence of high concentrations of water soluble stabilizers areused to produce latex-like dispersions of particles having smallparticle size. Such suspension polymerizations may be referred to asdispersion polymerizations. Inverse microsuspension polymerizationinvolves an organic solvent as a continuous phase of a water solublemonomer either neat or dissolved in water. Inverse dispersion refers tosystems involving the organic solvent as continuous phase with dissolvedmonomer initiator that yield insoluble polymer.

Emulsion polymerization involves the polymerization of monomers in theform of emulsions, i.e., colloidal dispersions. Emulsion polymerizationdiffers from suspension polymerization in the type and smaller size ofthe particles in which polymerization occurs, in the kind of initiatoremployed, and in the dependence of polymer molecular weight on reactionparameters. For most polymerization processes there is an inverserelationship between the polymerization rate and the polymer molecularweight. Large decreases in the molecular weight of a polymer can be madewithout altering the polymerization rate by using chain transfer agents.Large increases in molecular weight can be made only by decreasing thepolymerization rate, by lowering the initiator concentration, orlowering the reaction temperature.

Emulsion polymerization allows increasing the polymer molecular weightwithout decreasing the polymerization rate. Emulsion polymerization hasthe advantage of being able to simultaneously obtain both high molecularweights and high reaction rates. The dispersing medium is usually waterin which the various components are dispersed by means of an emulsifier.Other components include the monomer, a dispersing medium and a watersoluble initiator. Surfactants are typically used in emulsionpolymerizations at from 1 to 5% weight. The ratio of water to monomer isgenerally in the range 70/30 to 40/60 by weight.

The polymerization processes discussed above involve additional stepseither to dry the polymer formed, separate the polymer from the organicsolvent used in the process, or to recover the organic solvent. Theadded steps require additional energy and time in preparing the finalproduct, thereby increasing the cost of the polymer produced. Moreover,the polymers produced using these known processes typically are producedas an agglomeration which must, following drying, be pulverized or insome way broken up to yield a usable polymer product. Breaking up thepolymer product by grinding or pulverizing produces a substantial amountof dust which raises environmental and health concerns to those havingto work in and around the polymer dust.

Therefore, there remains a need for a polymerization process and anapparatus in which to carry out the process which will enable theproduction of a dry polymer particle powder, thus eliminating the needto dry and pulverize the polymer product produced. There is also a needfor a polymerization process for producing polymers which areimmediately available for use following the completion of thepolymerization process.

Finally, there is a need for a process to produce a polymer which allowsthe size, shape, and density of the polymer to be controlled easily andprecisely. This is particularly important, for example, with polymersused in situations where a smooth surface is beneficial, such as infiber optic cables. Fiber optic cables, which are becoming more commonin telecommunications, are susceptible to invasive water. However, theuse of super-absorbing polymers in fiber optic cables is problematicalbecause of the relatively “soft” cladding around each fiber. The“softness” of the cladding makes it prone to scratching, which altersthe refractive index of the cladding, and therefore, the ability of thefiber to conduct light. Experimentation has shown that superabsorbersproduced by known processes cause very fine scratching of the cladding,an effect which is attributed to the rough surfaces of the polymerparticles resulting from the pulverization, grinding, or chopping of thesolid cross-linked polymer resulting from the above-described productionmethods into a fine powder as described above. The scratches in thesurface of the cladding occur whenever the fiber optic cable is flexed,e.g., when the cable is wound on a spool and then wound off the spoolfor installation.

Polymerization processes frequently require that polymerization takeplace on substrate or on a nucleating particle of some kind. Forexample, U.S. Pat. No. 4,135,043 discloses a process for manufacturinghydrophilic polymers. A previously formed polymer is coated with similartype monomers to form a coating on the polymer seed. Thereafter, thecoated seed is heated in order to polymerize the coating thereon.Processes of this type also require that the additional production stepsdiscussed above be employed.

Exemplary of the shortcomings of current polymerization processes arethe known methods for the production of water-absorbing polymers. Suchmethods can be categorized as involving either an aqueous system or amulti-phase process. Aqueous systems for production of such polymersgenerally result in a semi-solid mass of material from which water mustbe removed in an energy-intensive drying step. For instance, in U.S.Pat. No. 4,295,987, the mixture of polymerized monomers must bedehydrated with excess methanol to form a firm solid that is dried in,for instance, a vacuum oven, and then ground into particles of a desiredsize or into a powder.

Also known are methods for continuous production of such polymers in anaqueous system as illustrated by the description set out in U.S. Pat.No. 4,525,527, hereby incorporated in its entirety by this specificreference thereto. Briefly, that patent describes the heating of anaqueous monomer solution to which an initiator is added by pouring theinitiator onto the mixture as the mixture flows onto a travelingconveyer belt. The polymerization is exothermic, helping to drive offthe water, and results in a “relatively dry, solid polymer of low watercontent”, said to be 8-15% water. The solid polymer is then made into apowder by pulverization.

Multi-phase processes involve polymerization of an aqueous reactionmixture in an inert organic solvent, followed by the removal of thesolvent from the polymerized product. So far as is known, such processesare batch processes, and a representative example process in U.S. Pat.No. 4,446,261, hereby incorporated in its entirety by this specificreference thereto. That patent describes the preparation of polymerbeads by suspension of an aqueous solution monomer and crosslinker in ahydrocarbon or halogenated aromatic hydrocarbon and polymerization ofthe monomer upon addition of a water soluble initiator. As described inthat patent, the hydrocarbon is removed by distillation under reducedpressure and the residual polymer particles dried by heating underreduced pressure.

Other examples of such processes are found in the following U.S.patents:

AQUEOUS 3,661,815 3,669,103 4,071,650 4,167,464 4,286,082 4,295,9874,342,858 4,351,922 4,389,513 4,401,795 4,525,527 4,552,938 4,612,2504,618,631 4,654,393 4,703,067 MULTI-PHASE 4,059,552 4,093,776 4,340,7064,446,261 4,459,396 4,666,975 AQUEOUS/MULTI-PHASE 4,062,817 4,654,039

Both types of processes are characterized by a number of disadvantageswhich add to the cost of producing such polymers such that there is aneed for an improved method for producing these and other polymers. Forinstance, both aqueous and multi-phase batch processes require drying ofthe polymer.

Another disadvantage to producing polymers by known polymerizationmethods, particularly with respect to water-absorbing polymers, is thedifficulty often experienced in controlling the size, shape, and densityof the polymers produced. For example, water-absorbing polymer particleswith a smooth external surface are, so far as is known by Applicants,are previously unknown. It appears that at least some multi-phasemethods for production of such polymers result in spherical (see, forinstance, column 5, lines 39, 48 and 60 of the above-incorporated U.S.Pat. No. 4,446,261) or donut-shaped (see column 6, line 57 of theabove-incorporated U.S. Pat. No. 4,342,858) particles, but Applicantshave been unable to find any such particles which have a smooth surface.Instead, all known particles are characterized by either a rough surfaceor by a surface such as that described in the above-listed U.S. Pat. No.4,342,858 (column 6, lines 56-58) as being “high surface area donuts ofcollapsed spherical shapes with 2 to 5 micron protuberances on theirsurfaces”.

Some additional disadvantages are characterized at, for instance, column2, lines 56 et seq. of U.S. Pat. No. 4,093,776 and column 1, lines 18-56of U.S. Pat. No. 4,625,001, both hereby incorporated in their entiretyby this separate reference thereto.

It is, therefore, an object of the present invention to provide a novelpolymer in which the size, shape and density of the polymer can beeasily and precisely controlled, and a process and apparatus for doingso.

It is another object of the present invention to provide a novel polymerin which the degree of crosslinking and the water content of theresulting particle can be conveniently and precisely controlled, and aprocess and apparatus for doing so.

It is an object of the present invention to provide a novel polymerwhich is not formed on a substrate or other precursor which acts as aseed or nucleus for the polymer formation, and a process and apparatusfor doing so.

It is an object of the present invention to provide a process andapparatus for producing a polymer which eliminates the difficultyinherent in the handling of the highly viscous solution, gel, or cakeresulting from the production of such polymers with known processes.

It is another object of the present invention to eliminate the necessarygrinding, pulverization, and/or chopping of a semi-solid mass of polymerwhich characterizes known processes for making polymers.

It is another object of the present invention to eliminate the costlystep of recovering the organic solvent or medium used in known processesfor the production of polymers.

Another object of the present invention is to provide a process andapparatus for producing a polymer which eliminates the costly dryingstep of many known processes for producing polymers.

It is an object of the present invention to provide a polymerizationprocess which allows the polymerization reaction, polymer particle size,polymer particle shape, and water content of the polymer particle powderproduced to be easily controlled.

It is an object of the present invention to provide a process andapparatus for using a fluid source of a selected monomer to produce asubstantially dry polymer particle powder.

Other objects, and the advantages, of the present invention will be madeclear to those skilled in the art from a review of the followingdetailed description of the presently preferred embodiments thereof.

SUMMARY OF THE INVENTION

An atmospheric chain polymerization process is provided. The processcomprises preparing a first mixture of at least one monomer source and asolvent selected from the group consisting of water, organic solventsand mixtures thereof, and adding an initiator source to the firstmixture to form a second mixture of polymerization reagents Acrosslinker, neutralizer, or other reagents may also be added to thefirst mixture. The mixture of polymerization reagents is sprayed into aheated, controlled atmosphere. The heated, controlled atmosphere ismaintained in a closed vessel, such as a reaction chamber. Spraying themixture results in the formation of droplets which experience free fallthrough the heated, controlled atmosphere for a sufficient period oftime to obtain a desired degree of polymerization. The solvent iscontinuously evacuated from the atmosphere during the polymerizationprocess.

It is preferred that the mixture of polymerization reagents is sprayedinto the chamber at the top section of the chamber. It is also preferredthat the reaction chamber is about 3.65 to about 30.48 meters in height.Preferably, the pressure in the reaction chamber is maintained at lessthan ambient atmospheric pressure. Desired pressure levels include fromabout 338.6 to about 50,790 N/m² below ambient atmospheric pressure.Further, it is preferred that the reaction chamber is heated to atemperature of from about 23.8° C. to about 148.8° C. during thepolymerization reaction.

It is further preferred that the first mixture is maintained at adesired temperature and pressure prior to adding the initiator source.It is also preferred that the second mixture is maintained at atemperature of between about 26.60 and about 93.3° C.

In the preferred embodiment of the process, the second mixture ofreagents is sprayed into the atmosphere through one or more nozzles. Asubstantially dry polymer particle powder is produced by thepolymerization reaction. Polymer particle is defined herein as asubstantially dry powder-like product. Substantially dry means that theparticles will not agglomerate or stick together, and the powder is freeflowing. The present process enables the production of polymers fromabout 20 microns to about 125,000 microns in size. However, polymersize, shape, and density in a powder will be fairly uniform whenprepared under similar pressure, temperature, and the other parameterswhich will be more fully discussed in the Detailed Description of theInvention.

The size of the polymer particle produced may be varied by increasing ordecreasing the size of the nozzle opening. In the preferred process themixture of polymerization reagents is sprayed into the heated,controlled atmosphere at a pressure of between about 517 KPa and about13.7 MPa. The dry polymer particle powder may be recovered whilemaintaining a heated, controlled atmosphere.

The monomer source may be an aqueous solution, slurry, or emulsion of aselected monomer. The preferred solvent is water. Similarly, thecrosslinker source may be an aqueous solution, slurry, or emulsion of aselected crosslinker and the solvent is water.

Monomer sources which may be polymerized using the process includeunfunctionalized olefins and functionalized olefins, including but notlimited to acrylic acid, ethylenes, dienes, styrenes, propylenes,vinyls, methacrylates, acrylonitrile, methacrylonitrile, acrylamide,methacrylamide, and alkenes.

A polymerization process is also provided for producing an acrylic acidcontaining polymer. The process comprises the steps of preparing anaqueous mixture of partially neutralized acrylic acid, adding apolymerization initiator to the aqueous mixture to form a second mixtureof polymerization reagents, and spraying said second mixture into aheated, controlled atmosphere. The spraying of the mixture results inthe formation of droplets of said second mixture which are allowed tofall through the heated, controlled atmosphere for a sufficient periodof time to obtain a desired degree of polymerization and thus form apartially neutralized acrylic acid containing polymer in particle powderform.

Polymers produced according to the process are also provided.

A radical polymerization apparatus is provided. The apparatus comprisesa reaction chamber, a means for heating the reaction chamber, preferablyfrom about 3.65 to about 30.48 meters in height, a means for controllingthe pressure within said reaction chamber, one or more nozzles connectedto a feed line for receiving a mixture of a monomer source and aninitiator source, and a means for spraying the mixture into the topsection of the reaction chamber. spraying the mixture into the chamberresults in the formation of droplets which fall through the reactionchamber for a sufficient period of time to obtain a desired degree ofpolymerization. It is preferred that the mixture is sprayed into thereaction chamber at from about 517 KPa to about 13.7 MPa of pressure.

Preferably, the pressure in the reaction chamber is maintained at lessthan ambient pressure. Desired pressure levels include from about 338.6to about 50,790 N/m² below ambient atmospheric pressure. Further, it ispreferred that the reaction chamber is heated to a temperature of fromabout 23.8° C. to about 148.8° C. during the polymerization reaction.

The apparatus may further comprise a means for removing the polymer fromthe bottom of said reaction chamber while maintaining the heat and thepressure within the chamber. The means for removing the polymer maycomprise a trough located in the bottom of the reaction chamber, a firstauger mounted in the bottom of the trough for moving the polymer out ofthe chamber, a bin into which the first auger deposits the polymer and asecond auger mounted in the bottom of the bin for moving the polymer outof the bin.

It is preferred that the apparatus further comprises a means forcontrolling the temperature of the monomer and initiator mixture in thefeed line. It is also preferred that the apparatus includes a means formixing the monomer and initiator before it is sprayed into the chamber.

Means are also provided for removing unpolymerized monomer in a vaporstate from said reaction chamber by purging the reaction chamber withnitrogen. Separate reservoirs are provided for the monomer source andthe initiator source. Reservoirs may also be provided for a crosslinkersource, and other reagents. When it is desired that crosslinked polymerstructures be produced, crosslinker is added to the mixture of monomersource and initiator source prior to the mixture being sprayed into thereaction chamber.

Polymers produced using the apparatus may be polymerized fromunfunctionalized olefins and functionalized olefins, including but notlimited to acrylic Acid, ethylenes, dienes, styrenes, propylenes,vinyls, methacrylates, acrylonitrile, methacrylonitrile, acrylamide,methacrylamide, and alkenes.

A polymer is produced by a radical polymerization process comprising thesteps of preparing a first mixture of at least one monomer source and asolvent selected from the group consisting essentially of water andorganic solvents, adding an initiator source to the first mixture toform a second mixture of polymerization reagents, then spraying thesecond mixture of reagents into a heated, controlled atmosphere. Theheated, controlled atmosphere is maintained in a closed vessel orreaction chamber which is preferably about 3.65 to about 30.48 meters inheight. Preferably, the second mixture is sprayed in at the top sectionof the vessel.

It is preferred that the first mixture is maintained at a desiredtemperature and pressure prior to adding the initiator source. It isalso preferred that the second mixture is maintained at a temperature ofbetween about 26.6° C. and about 93.3° C. The spraying of the secondmixture of reagents into the atmosphere through one or more nozzlesresults in the formation of droplets which are allowed to fall throughthe heated, controlled atmosphere for a sufficient period of time toobtain a desired degree of polymerization. The solvent and unpolymerizedmonomer is continuously evacuated from the atmosphere during thepolymerization process. The polymer produced is in dry particle powderform and is recovered from the vessel while maintaining a heated,controlled atmosphere. The polymers may range in size from about 2 toabout 125, 000 microns in size. The size of the polymer particleproduced can be varied by increasing or decreasing the size of thenozzle opening. It is preferred that the second mixture ofpolymerization reagents be sprayed into the heated, controlledatmosphere at a pressure of between about 517 KPa and about 13.7 MPa.

In a preferred embodiment, the atmosphere is maintained at a temperatureof from about 23.8° C. to about 148.8° C. It is also preferred that thecontrolled atmosphere is at reduced pressure maintained at a pressure offrom about 338.6 to about 50,790 N/m² below ambient atmosphericpressure.

When desired to produce crosslinked polymer structures, a crosslinkermay be added to the first mixture. A neutralizer may also be added tothe first mixture. The monomer source may be a water-soluble,unsaturated monomer which, when a crosslinker is added to thepolymerization mixture of reagents, results in a water-absorbing polymerbeing produced.

The monomer source may be an aqueous solution, slurry, or emulsion of aselected monomer. The preferred solvent is water. Similarly, thecrosslinker source may be an aqueous solution, slurry, or emulsion of aselected crosslinker and the solvent is water.

The monomer source may be unfunctionalized olefins or functionalizedolefins, including but not limited to acrylic acid, or selected from thegroup of compounds consisting essentially of ethylenes, dienes,styrenes, propylenes, vinyls, esters, acrylates, methacrylates,acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, andalkenes.

An acrylic acid containing polymer is produced by an aqueous radicalpolymerization process comprising the steps of preparing an aqueousmixture of acrylic acid, adding a polymerization initiator to theaqueous mixture to form a second mixture of polymerization reagents, andspraying the second mixture into a heated, controlled atmosphere.Spraying results in the formation of droplets of the second mixture. Thedroplets of the sprayed second mixture are allowed to fall through theheated, controlled atmosphere for a sufficient period of time to obtaina desired degree of polymerization and thus form an acrylic acidcontaining polymer in particle powder form. The polymer produced is awater-absorbing polymer in substantially dry powder form and has, forexample, a particle size of less than approximately 100 microns.

An unexpected advantage of producing polymers in accordance with thepresent invention is the production of a particle having a smoothexternal surface, making possible certain new uses for such polymers,particularly water-absorbing polymers. For instance, known waterabsorbing polymers may be used to advantage in telecommunications cablesfor the purpose of protecting the conductors from shorts caused byinvasive water.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an apparatus constructed inaccordance with the present invention.

FIG. 2 is a sectional view, taken along the lines 2'2 in FIG. 1, of theapparatus of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, there is shown an apparatus constructed inaccordance with the present invention designated generally at referencenumeral 10. Briefly, the apparatus 10 is comprised of a reaction chamberor other closed vessel 12, means for heating the reaction chamber 12,shown schematically at reference numeral 14, means for reducing thepressure in reaction chamber 12 in the form of the vacuum pump 16,nozzle 18 connected to a feed line 20 for receiving a mixture of amonomer and an initiator from the respective sources 22 and 28 thereof,and means, in the form of the bins and auger system indicated generallyat reference numeral 26, for removing the dry polymer particle powderproduced in reaction chamber 12 from the bottom thereof whilemaintaining reduced pressure in the reaction chamber 12. Reactionchamber 12 will preferably be about 3.65 to about 30.48 meters inheight.

In more detail, the apparatus 10 includes sources, or reservoirs, ofmonomers 22, initiator 28, and a third reservoir 24 preferablycomprising a source of a crosslinker. A crosslinker or combination ofcrosslinkers will be added to the monomer and initiator mixture when. itis desirable to produce crosslinked polymer structures, such aswater-absorbing acrylic family polymers. Other reservoirs (not shown),each preferably individually temperature controlled, are provided forother monomers (for co-polymerization processes), water, neutralizer,stabilizer, transfer agent, solvent, or other reagents.

Each of the reservoirs 22, 24 and 28 may contain an aqueous and/ororganic solvent-based solution, suspension, or emulsion of therespective reagent and therefore is preferably provided with valves 30,32 and 34, respectively, for controlling the flow of the respectivereagents therefrom under the influence of the pumps 36. Alternatively,feed line 20 is pressure fed from the respective reservoirs 22, 24 and28. Feed line 20 is provided with means., indicated generally atreference numeral 44, for mixing the aqueous and/or organicsolvent-based mixture of reagents. to insure uniform distributionbetween monomer, initiator, and other reagents of the polymerizationmixture. Mixing means 44 takes a number of forms known in the art suchas an in-line screw or auger, mill, vat with stirring blades, or bafflesystem. Also provided is a hatch 38 in the housing of mixing means foradding any other types of reagents.

Because the polymerization of the monomer begins almost instantaneouslyupon the mixing of the initiator and monomer and proceeds rapidly in anexothermic reaction, the temperature of the mixture in feed line 20 isused to control the degree of polymerization of the monomer and, whencrosslinkers are added, the degree of crosslinking of the polymerstructure (note that FIG. 1 is not drawn to scale). To that end, a meansis provided for controlling the temperature of the mixture in feed line20 which is shown schematically at reference numeral 42. Temperaturecontrol means 42 may take the form of a water bath, refrigeration coils,insulation, a combination of heating elements and refrigeration coils,or any other means known for controlling temperature, depending upon thenature of the monomer(s) being polymerized and/or crosslinked. Means 42is used primarily to reduce the heat resulting from the exothermicpolymerization reaction. Lowering the temperature of the reagent mixturein line 20 prevents the polymerization rate of the monomer source.

In a preferred embodiment, line 20 is provided with a nitrogen purechamber 45 fed by a line 46 having a valve 48 therein for a flow ofnitrogen to purge the mixture of reagents flowing therethrough. Ametered pump 40 in feed line 20 provides control over the pressure andrate of flow of the mixture of reagents.

The pressure of the atmosphere in reaction chamber 12 is controlledduring the polymerization reaction. The pressure of the atmosphere inreaction chamber 12 may be reduced relative to the ambient pressure byvacuum pump 16, the atmosphere therein being removed from reactionchamber 12 through line 50. A filter 52 is provided at the intake ofline 50 to prevent droplets/particles of polymerized monomer from beingpulled into vacuum pump 16. Filter 52 is preferably set in a frameaccessible from the exterior of reaction chamber 12 to facilitateperiodic changing of the filter element (not shown).

The pressure of the atmosphere in reaction chamber 12 may also beincreased above ambient atmospheric pressure in order to produce apolymer particle of a more dense construction than is produced at belowatmospheric pressure.

An air intake or vent line 54 having a flow control valve 56 therein isprovided at the top of reaction chamber 12 for allowing ambient air intoreaction chamber 12 to purge line 50 and pump 16 and/or in the event ofan emergency shutdown. Instead of using air intake line 54 as a vent,nitrogen can also be introduced into the atmosphere in reaction chamber12 through line 54 to purge the atmosphere therein, in which case, apurge line 55 is provided for recapturing the nitrogen. Again, a filter57 prevents polymer particles from being pulled into line 55. Reactionchamber 12 is also provided with means 62, in the form of a clean-outhatch, for removing unpolymerized monomer, or, in the event of amalfunction of particle removing means 26, the dry polymerizedparticles, from the bottom thereof. Heater 14 may be multiple resistiveheating elements, gas burners, or steam lines, indicated schematicallyat reference numeral 58, powered from a common line 60 from a source ofelectricity, steam, or gas (not shown).

Means 26 is provided for removing the polymer from the bottom ofreaction chamber 12 while maintaining the heat and atmosphere therein.Means 26 may be any of the devices known to those skilled in the art.For example, as shown in FIG. 1, a V-shaped trough 64 is formed from twosloping sides 66 which form the bottom of reaction chamber 12. A firstauger, screw, or other continuous conveyer system 68 is mounted in thebottom thereof for moving the particles of polymer out of reactionchamber 12 for deposit into a bin 70. Bin 70 is provided with a secondauger, screw, or other continuous conveyer system 72 mounted in thebottom thereof for moving the polymer deposited therein by first auger68 out of bin 70 into a hopper, bag or other storage/collectioncontainer 74. The respective augers 68 and 72 are driven by motors 76and 78 or by a single motor and belt pulley system (not shown).

The accumulated dry polymer particles 80 in the bottom of tower 12 coverthe portal 82 out of the bottom reaction chamber 12 to maintain thereduced pressure therein as first auger 68 moves the accumulated polymerparticles 80 into bin 70. A shroud 84 covers first auger betweenreaction chamber 12 and bin 70 to prevent the influx of ambient air intoreaction chamber 12 and bin 70 is closed by a lid 86 at the top thereof.A portal 88 in the bottom of bin 70 allows the accumulated polymerparticles in bin 70 to be moved out of bin 70 into hopper 74. In thesame manner that the accumulated polymer particles 80 cover the portal82 in the bottom of reaction chamber 12, accumulated polymer particles90 cover the portal 88 in the bottom of bin 70 to prevent an influx ofambient air therethrough.

As will be explained below, the conditions under which polymerizationand crosslinking occur are such that the particles of polymer 80 whichaccumulate at the bottom of reaction chamber 12 are substantially dryand have a low moisture content, and in light of the ability of somepolymers to absorb moisture, preventing access to the accumulatedpolymer 80 by relatively humid ambient air is necessary to preventingthe caking of the dry, accumulated polymer 80 as it is moved out ofreaction chamber 12. Those skilled in the art who have the benefit ofthe disclosure will recognize that other means can be utilized to removethe dry accumulated polymer 80 from the bottom of reaction chamber 12without breaking the vacuum therein. For instance, commerciallyavailable gravity feed, intermittent dump, and suction devices are allused to advantage for this purpose.

As shown in FIG. 2, feed line 20 enters reaction chamber 12 andterminates in a loop 94 having a plurality of nozzles 18 set therein.Alternatively, a single nozzle may be used. Nozzles 18 are preferablyscrewed into the threads 96 formed at intervals around the loop 94 forease in removing nozzles 18 for cleaning and/or changing. In anotherpreferred embodiment (not shown), the loop 94 in feed line 20 ispositioned outside of the top of tower 12. The latter embodiment isparticularly advantageous for frequent changing of nozzles 18 to, forinstance, change the diameter of the polymeric spheres.

If larger diameter particles are desired, the mixture in line 20 issprayed into reaction chamber 12 at relatively low pressure, e.g., about517 KPa to about 2.06 MPa through nozzles 18 having a larger diameterorifice, for instance, a number 16 nozzle. If it is desired to produce asmaller diameter sphere, the pressure in line 20 is increased with pump40, e.g., from about 1.03 MPa on up to as high as perhaps 13.7 MPa,causing the mixture to be atomized into finer sized particles. If evensmaller particles are desired, or if it is desired to produce smallerdiameter particles at lower spray pressures, nozzles 18 are replacedwith nozzles having smaller orifices, e.g., a number 64 nozzle.

With reference to the figures, the method of continuously producing apolymer in accordance with the present invention will now be explained.The method comprises the mixing of the monomer source in water ororganic solvent in mixer 44 to prepare a liquid monomer source.Alternatively, liquid monomer sources may be obtained from sources knownto those skilled in the art. It is preferred that the monomer be anaqueous monomer source. An initiator, crosslinker, additional water,neutralizer and/or other reagents may be added to the monomer source inmixer 44. In the preferred embodiment of the process, the monomer,initiator, and other reagents used will be soluble in water or theorganic solvent used to prepare the liquid monomer source. It ispreferred, however, that the monomer, initiator and other reagents aresoluble in water. Initiator sources suitable for use with selectedmonomers will be known to those skilled in the art.

It is preferred that the initiator is added downstream from the monomersource immediately prior to the mixture being sprayed into the reactionchamber 12 to reduce the likelihood that line 20 or nozzles 18 willbecome clogged by prepolymerization of the monomers. Although it ispreferred that an initiator be added to the monomer source, somemonomers may undergo self-initiated polymerization, thus eliminating theneed for an initiator. However, the rate and extent of polymerization isfacilitated by the use of an initiator.

Note that it may be advantageous to agitate or otherwise insure adequatemixing of initiator and aqueous mixture by use of a second mixing means(not shown) similar to mixing means 44 downstream of the point at whichthe initiator is added to line 20.

Reaction chamber 12 is heated by heater 14 and the pressure of theatmosphere is controlled according to the monomer source beingpolymerized. Desired pressures at which to cause polymerization ofselected monomers will be known to those skilled in the art. Theatmosphere is maintained at a reduced pressure by vacuum pump 16. Theatmosphere may be controlled at a positive pressure by use of a pressurerelease valve in conjunction with the pressure evacuation pump. However,a positive pressure may be established by a pressure pump or other meansknown to those in the art in the chamber prior to spraying in themixture.

The mixture of monomers and/or reagents is sprayed into the heated,controlled atmosphere, thereby forming droplets of the mixture. Thedroplets are allowed to fall through the reaction chamber for a periodof time sufficient to allow the desired degree of polymerization of themonomers, and where a crosslinker has been added, crosslinking of thepolymerized monomer, to form particles of polymer. The polymer particlesformed may be linear, branched or crosslinked polymers, depending uponthe composition of the sprayed mixture.

As the droplets of the mixture of monomers and/or reagents fall from thespray nozzle through the chamber, water and/or solvents associated withthe polymerization mixture are continually evaporated or volatilized asthe polymerization reaction occurs. In the presence of the preferredreduced pressure within chamber 12, water and/or solvent associated withthe mixture is continually evaporated or volatilized due to both thepresence of heat and the reduced pressure. Vapors are evacuated byvacuum pump 16 during the polymerization action. Alternatively, in thepresence of positive pressure within chamber 12, the water and/orsolvent associated with the mixture is evaporated due to the heat withinthe chamber.

The polymer particles produced as a result of the polymerization processare in the form of a substantially dry powder 80. The polymers producedin dry particle form by this process may have a particle size from assmall as approximately 2 microns up to as large as about 125,000 microns(approximately ⅛ inch). The size of the polymer will depend upon thepressure, temperatures, and other parameters discussed more fully belowwhich are present and/or utilized during the polymerization process. Thepolymer size, shape, and density, however, will be relatively uniformwhen produced under similar conditions. The fallen particles of polymer80 which collect at the bottom of reaction chamber 12 are removed whilecontinuing to control the pressure within the chamber. The atmospherewill be maintained at the pressure desired for the selectedpolymerization desired as the polymer is removed.

In a particularly preferred embodiment, the method of the presentinvention additionally comprises allowing sufficient fallen particles ofpolymer 80 to accumulate at the bottom of reaction chamber 12. Theaccumulated polymer particles form a large enough mass of heatedparticles to decrease the amount of energy required to maintain thetemperature of the atmosphere within reaction chamber 12 at a desiredtemperature.

It is preferred that the atmosphere in reaction chamber 12 be heated toa temperature of between about 23.8° C. and about 148.80° C.Temperatures of between about 82.20° C. and about 110° C. have beenfound to be particularly preferred. Although it is possible to makepolymers at temperatures below 37.8° C., a larger reduction of thepressure within reaction chamber 12 must be present if low water orsolvent content and low residual monomer levels are to be maintained.

The polymerization reaction and, when crosslinker is added to themixture, crosslinking which occurs as the droplets fall through reactionchamber 12 is exothermic such that relatively little external heat isneeded from heater 14 once a desired temperature is reached and thecontinuous polymerization/crosslinking is underway. Hence, temperaturesof well above, for instance, 37.8° C. can be maintained withoutcompromising the efficiency of the method. Further adding to theefficiency of the method of the present invention is the heat retentionof the accumulated mass of particles 80 at the bottom of reactionchamber 12.

The pressure of the atmosphere inside reaction chamber 12 need not bereduced below or raised above ambient atmospheric pressure to producepolymers. However, even relatively minor reductions in the pressurebelow nominal ambient pressure of about 101,580 N/m² causedisproportionate decreases in the water content of the polymers producedand the amount of unpolymerized residual monomer which does notparticipate in the polymerization and/or cross-linking reaction. Forthis reason it is preferred that the polymerization process be conductedunder reduced pressure conditions in the chamber.

Reductions in pressure of from about 338.6 up to about 50,790 N/m² ispreferred for use in connection with the method of the presentinvention. For a given particle size, the water and/or residual monomercontent is directly correlated between the temperature of the atmospherewithin reaction chamber 12 and the pressure at which the atmospherewithin reaction chamber 12 is maintained. As a general rule, astemperature is increased, the percentage of residual monomer and/orwater or solvent content can be maintained at low levels withprogressively smaller reductions in pressure relative to ambientpressure. Likewise, as the pressure of the atmosphere in reactionchamber 12 is decreased relative to ambient, low percentages of residualmonomer and/or water or solvent content can be maintained withprogressively lower temperatures.

Alternatively, the polymerization reaction may be conducted underincreased pressure conditions within reaction chamber 12. Increases inpressure within the reaction chamber of from about ambient to about 2psi may be used to practice the invention. As a general rule, astemperature is increased, the percentage of residual monomer and/orwater or solvent content can be maintained at desired low levels withprogressively smaller increases in pressure relative to below ambientpressures. Likewise, as the pressure of the atmosphere in reactionchamber 12 is increased relative to ambient, low percentages of residualmonomer and/or water or solvent content can be maintained withprogressively higher temperatures.

The shape of the polymer is also influenced by the below ambientpressure in the chamber. Varying the pressure allows production of fromspherical to more irregular-shaped polymers. At lower below ambientpressures, the polymers will be less uniform in shape and have rougheredges or flake-like appearances. As the reduced pressure approachesambient, the polymers become more spherical than is observed withpolymers produced, for example, at 15 inches of Hg below atmosphericambient pressure. Increasing the pressure within the chamber enables theproduction of a smoother, more dense polymer sphere than those producedin low atmospheric pressure conditions.

The period of time required during which the droplets fall through theatmosphere within reaction chamber 12, determine the height of thereaction chamber. As a general rule, fall times of between about 5 andabout 60 seconds are required under the preferred conditions oftemperature and controlled pressure to achieve the desired degree ofpolymerization, crosslinking, and evacuation of water/solvent andresidual monomer for most monomer/crosslinker mixtures. Experimentationhas shown that reaction chamber height must be adequate to allowsufficient reaction chamber fall times in order to achieve the desiredextent of polymerization.

The period of time the droplet is allowed to fall through reactionchamber 12 (and hence, the height of that chamber) is also related inpart to the diameter of the droplet sprayed from nozzle 18. A nozzle 18having a large orifice 98 therein produces a larger diameter dropletsuch that either the reaction chamber fall time of the droplet may beincreased, the reduction in the pressure of the atmosphere in reactionchamber 12 may be increased, the temperature may be increased, or somecombination of changes in these three parameters may be effected, inorder to obtain the desired degree of polymerization, and water/solventevaporation throughout the resulting particle. In the case of very largediameter droplets, pressure reductions of greater than 67,720 N/m²,temperatures of over 101.7° C., and reaction chamber fall times in the20 second or higher range may be required to allow the desired degree ofpolymerization and evaporation of liquid.

Another variable affecting the production of polymers in accordance withthe method of the present invention is the pressure at which the mixtureof monomer and initiator and/or crosslinker is sprayed into the heated,controlled atmosphere in reaction chamber 12. In general, as thepressure at which the mixture is sprayed increases, the size of thedroplet decreases, requiring concomitant changes in either reactionchamber fall time, temperature, reduction in pressure in reactionchamber 12, the diameter of the orifice 98 in nozzle 18, or somecombination thereof. Satisfactory results have been obtained at spraypressures of between about 517 KPa and about 13.7 MPa, and pressures ofbetween about 698.3 KPa and about 1.20 MPa represent a particularlypreferred embodiment of the method of the present invention. Thepressure in the line at which the mixture is sprayed, as well as thereduced temperature, prevent the polymerization of monomers prior totheir being sprayed into the chamber. Therefore, the combination ofdecreasing temperature in the feed line, increasing pressure in the feedline and introducing the initiator immediately prior to spraying themixture into the chamber essentially eliminates any prepolymerization ofmonomer in the feed line.

A final variable affecting the method of the present invention and thevarious parameters of pressure, temperature and fall time at which themethod is conducted is the nature of the monomer, initiator, crosslinkerand other reagents desired to be polymerized. By reference herein to amonomer, it is intended to refer to any organic molecule which iscapable of being polymerized by covalent bonding to other organicmolecules and/or itself to form chains. Many of the polymers can also becrosslinked. Although it is not intended to limit the method of thepresent invention to only the following monomers, the following listingwill provide an example of certain monomers, including functionalizedand/or unfunctionalized olefins, which may be polymerized and in mostcases crosslinked in accordance with the method of the presentinvention:

(a) water-soluble, ethylenically unsaturated monomers as described inthe above-incorporated Yamasaki, et al. U.S. Pat. No. 4,446,261: acrylicacid, methacrylic acid, salts of acrylic acid and methacrylic acid,acrylamide, methacrylamide, N-substituted acrylamides, N-substitutedmethacrylamide, 2-acryloylethane-sulfonic acid, 2-methacryloylethanesulfonic acid, salts of 2-acryloylethane-sulfonic acid and2-methacryloylethane-sulfonic acid, styrene-sulfonic acid, salts ofstyrene-sulfonic acid, 2-hydroxyethyl acrylate and2-hydroxyethylmethacrylate;

(b) the monomers described in Markus, U.S. Pat. No. 2,810,716: acrolein,allylidenediacetate, acrylonitrile, esters of acrylic and methacrylicacid (including methyl methacrylate, ethyl methacrylate, fumaric acid,monomethylfumarate, dimethylfumarate, monoethylfumarte, diethylfumarte,maleic anhydride, maleic acid, monomethylmaleate, dimethylmaleate,monoethylmaleate, diethylmaleate, dimethylmethylenemalonate,diethylmethylene malonate, itaconic acid, monomethylitaconate, dimethylitaconate, monoethyl itaconate, diethylitaconate, atrophic acid, methylatropate, and ethyl atropate), chloracrylic acid and esters thereof,bromoacrylic acid and esters thereof, iodoacrylic acid and estersthereof, ortho-, meta-, and paramethylstyrene, fluorostyrene andchlorostyrene, a-sulfoacrylic acid, salts and esters, a-amino-acrylicacid, salts and esters, n-monomethyl and N,N′-dimethyl acrylamide,acrylic and methacrylic anhydride, methylvinylketone,hydroxymethylvinylketone, ortho- and paramethoxystyrene, ethyleneglycolmonomaleate, ethylglycol monofumarate, N-vinylmethylacetamide, vinylacetate, vinyl butyrate, vinyl benzoate, vinylquinoline, andvinylpyridines such as 2-vinylpyridine, 4-vinylpyridine,2-methyl-5-vinylpyridine, 2-vinyl-5-ethylpyridine, N-vinylpyrrolidone,cyclopentadiene, N-vinylphthalimide, N-vinylsuccinimide,N-vinylacetamide and N-vinyl-diacetamide;

(c) the monomers described in Glavis, et al., U.S. Pat. No. 2,956,046:salts of unsaturated monomeric acids such as quaternary ammonium saltsand amine salts thereof and salts of ammonia, alkali metals and alkalineearth metals, including B-hydroxyethyltrimethylammonium acrylate,benzyltrimethyl ammonium acrylate, amine salts, monomethylammoniumacrylate, and di- and tri-methylammonium acrylate;

(d) the monomers described in Wichterle, et al., U.S. Pat. No.3,220,960: dimethylaminoethyl methacrylate, piperidinoethylmethacrylate, morpholinoethyl methacrylate, methacrylylglycolic acid,methacrylic acid, monomethacrylates of glycol, glycerol and ployhydricalcohols, dialkylene glycols and polyalkylene glycols and thecorresponding acrylates;

(e) the monomers described in Bashaw, et al., U.S. Pat. No. 3,229,769:cross-linked, substantially water-insoluble, water-swellable sulfonatedalkaryl and aromatic polymers such as cross-linked polysodium styrenesulfonate and sulfonated polyvinyltoluene salts, copolymers of suchsulfonated alkaryl and aromatic materials with acrylonitriles, alkylacrylonitriles, acrylates and methacrylates, cross-linked polyvinylalcohol and polyacrylamide and cross-linked copolymers ofpolyacrylamide, e.g., of acrylamide and acrylic acid or acrylamide andmonovalent salts of acrylic acid, and cross-linked heterocyclic monomerssuch as polyvinyl morpholinone, poly-5-methyl-N-vinyl-2-oxazolidinone,and polyvinyl pyrrolidene;

(f) the monomers described in Assarsson, U.S. Pat. No. 3,664,343:water-insoluble hydrophilic poly (ethylene oxide) polymers made bysubjecting water-soluble poly (ethylene oxide) polymers to ionizingradiation (such polymers are homopolymers of ethylene oxide andcopolymers of ethylene oxide with one or more alkylene oxides such aspropylene oxide, styrene oxide, and 1,2-butylene, 2,3-butylene andisobutylene oxide, all as described in U.S. Pat. Nos. 3,127,371,3,214,387, 3,275,998, 3,275,199 and 3,399,149);

(g) the monomers described in Gross, et al., U.S. Pat. No. 3,926,891:alkyl acrylates and methacrylates such as methyl acrylate, ethylacrylate, propyl acrylate, hexyl acrylate, butyl methacrylate, hexylmethacrylate, octyl methacrylate, decyl methacrylate and omega hydroxyalkyl acrylates such as 2-hydroxyethyl acrylate, hydroxymethyl acrylate,3-hydroxypropyl acrylate and 4-hydroxybutyl acrylate;

(h) ethylene, 1,3-dienes, styrene, halogenated olefins, vinyl esters,acrylates, methacrylates, acrylonitrile, methacrylonitrile, acrylamide,methacrylamide, N-vinyl carbazole, N-vinyl pyrrolidone, propylene, and

(i) alkenes, vinyl acetate.

It will be understood by those skilled in the art who have the benefitof this disclosure that the references to a monomer throughout thisspecification and claims also contemplate the co-polymerization ofmonomers such as maleic acid and styrene to form commercially usefulco-polymers, graft polymers, block polymers. Examples of suchco-polymerizations can be found, for instance, at column 2, lines 66 et.seq. of U.S. Pat. No. 4,057,521. As is the case for the selection of aparticular combination of monomer, crosslinker and initiator forpolymerization and cross-linking in accordance with the method of thepresent invention, the selection of such combinations of monomers iswell known in the art and forms no part of the invention hereof suchthat further description of such co-polymerizations is unnecessary.

In the same manner that the method of the present invention is notrestricted with respect to the particular monomer or monomers utilized,a large number of crosslinkers are utilized to advantage. Suchcrosslinkers include any organic compound capable of reacting with anorganic polymer in aqueous solution and include, generally, compoundshaving at least two polymerizable double bonds, compounds having atleast one polymerizable double bond and at least one functional groupreactive with an acid-containing monomer or polymer, compounds having atleast two functional groups reactive with an acid-containing monomer orpolymer, and polyvalent metal compounds which are capable of formingionic cross-linkages. A non-limiting listing of a number of suchcrosslinkers includes:

polyvinyls, e.g., divinylbenzene, divinyltoluene, divinyl acidanhydrides, divinyl sulfone, divinyl benzene sulfonate, and their alkylor halogen-substituted products;

polyesters of unsaturated mono- or poly-carboxylic acids with polyols,e.g., ethylene glycol, trimethylol propane, glycerine, andpolyoxyethylene glycols;

bisacrylamides, e.g., N,N′-methylene-bisacrylamide andN,N′-methylenebismethacrylamide;

carbamyl esters obtained by reacting polyisocyanates with hydroxyl-groupcontaining monomers;

di-, tri-, or tetraesters of acrylic or methacrylic acid;

polyallyl esters of polycarboxylic acids, e.g., diallyl phthalate anddiallyl adipate;

esters of unsaturated mono- or polycarboxylic acids with mono-allylesters of polyols, e.g., the acrylic acid of polyethylene glycolmonoallyl ether, allyl acrylate, diallyl ethylene glycol ether, anddivinyl ether of ethylene tri- or diethylene glycol;

di- or triallylamine, N,N′-diallylacrylamide, diallylmethacrylamide;

N-methylol acrylamide, N-methyloyl-methacrylamide;

glycidyl acrylate and methacrylate, polyethylene glycol diacrylate,polyethylene glycol dimethacrylate;

substituted hexadienes such as 2,5-dimethyl-3,4-dihydroxy-1,5-hexadieneand 2,5-dimethyl-2,4-hexadiene;

olefinically unsaturated mono- or polycarboxylic acids such as acrylic,methacrylic, crotonic, isocrotonic, angelic, tiglic, senecioic, maleic,fumaric, itaconic, aconitic, teraconic, citraconic, mesaconic, andglutaconic acid;

glyoxal;

polyols, e.g., ethylene glycol and polyhaloalkanols such as1,3-dichloroisopropanol and 1,3-dibromoisopropanol;

polyamines, e.g., alkylene diamines (ethylene diamine), polyalkylenepolyamines, triethanolaminediacrylate and dimethacrylate,triethanolamine triacrylate and trimethacrylate, and diacrylate anddimethacrylate of bishydroxylacetamide;

polyepoxides and haloepoxyalkanes, e.g., epichlorhydrin, epibromohydrin,2-methyl epichlorhydrin and epiiodohydrin;

polyglycidyl ethers and polyol polyglycidyl ethers such as ethylene,diethylene, and propylene glycol diglycidyl ether, glycerin triglycidylether, glycerin diglycidyl ether and polyethylene glycol diglycidylether, tartaric acid diacrylate and trimethacrylate, triethylene glycoldiacrylate and dimethacrylate, ethylene glycol dimethacrylate andpropylene glycol diacrylate;

oxides;

hydroxides;

weak acid salts (e.g., carbonate, acetate) of alkaline earth metals(calcium, magnesium) and zinc, strontium and barium, e.g., calcium oxideand zinc diacetate; and

polyvalent metal salts of acrylic acid and methacrylic acid.

Likewise, it is not intended that the method of the present invention berestricted with respect to the initiator utilized. Radicals can beproduced by a variety of thermal, photochemical, and redox(oxidative-reduction) methods known to those skilled in the art. Thebenefits of initiator selection are also known to those skilled in theart. Appropriate polymerization initiators well known in the artinclude:

peroxygen compounds (sodium, potassium and ammonium persulfate),hydrogen peroxide, caprylyl and benzol peroxide, cumene hydroperoxides,acetyl peroxide, tert-butyl diterphthlate, tertbutylperbenzoate, sodiumperacetate, tertbutylhydroperoxide, sodium percarbonate, andconventional redox initiator systems such as are formed by combining aperoxygen compound with a reducing agent such as sodium or ammoniumsulfite or bisulfite, L-ascorbic acid, or ferrous salts;

peroxides in combination with a reducing agent;

tetraphenylsuccinodinitrile, tetra-p-methoxyphenylsuccinodinitrile; and

azo initiators such as azobisiso-butyronitrile (AIBN),4-t-butylazo-4′-cyano-4,4′-azobis (4-cyanovaleric acid), 2,2′-azobis(2-amidino-propane)-hydrochloric acid salt.

It is also common to use mixtures of one or more of such initiators.

The present invention can be better understood by reference to thefollowing, non-limiting examples of cross-linked, water-absorbingpolymers produced in accordance with the method described above.

EXAMPLE 1

Aqueous solutions of the initiators sodium persulfate and 2,2′-azobis(2-amidinopropane HCl (ABAH) (Polysciences, Inc., Warrington, Pa.) andascorbic acid were prepared in the following proportions (all parts byweight):

initiator water sodium persulfate 0.650 12.346 ABAH 0.975 12.021ascorbic acid 0.013 12.983

Other reagents were used in the following proportions:

water 29.70% acrylic acid (Catalog No. 42.61% 14723-0, Aldrich ChemicalCo., Milwaukee, WI) triallylamine (Catalog No. 0.30% T4500-4, AldrichChemical Co., Milwaukee, WI) caustic soda 22.12% ascorbic acid solution1.05% ABAH solution 2.11% sodium persulfate solution 2.11%

The water and acrylic acid were mixed while holding the temperature at10° C. and the triallylamine added. Caustic soda was added at a rateslow enough to hold the temperature under 40° C., then temperature wasbrought down to 7° C. and held at that temperature as the ascorbic acidand ABAH solutions were added. The resulting aqueous mixture was purgedwith nitrogen for 4 to 5 minutes and the sodium persulfate added. Thatmixture was mixed for 30 seconds at a temperature under 37.8° C. Themixture was then sprayed into a reaction chamber constructed inaccordance with the teachings of the present invention that was about4.57 meters high with an atmospheric pressure therein of about 33,860N/m² below ambient and which had been heated to about 107.2° C. Theresulting water-absorbing spherical particles were smooth surfaced,ranged between about 50 to about 100 microns in diameter, and had awater content of about 2%.

EXAMPLE 2

The method described in Example 1 was modified by using the followingproportions of reagents:

water 26.00% acrylic acid (see Example 1) 44.85% triallylamine (seeExample 1) 0.31% caustic soda 23.29% ascorbic acid solution 1.11% ABAHsolution 2.22% sodium persulfate solution 2.22%

All process parameters were the same as in Example 1 except that thetemperature of the reaction chamber 12 was 65.6° C. and the pressure ofthe atmosphere in the reaction chamber 12 was not reduced below ambientpressure. Although the resulting spherical particles were smoothsurfaced and capable of efficient water absorption, water content wasabout 8%. When repeated using potassium hydroxide in place of causticsoda, yield dropped by about 50%.

EXAMPLE 3

The method described in Example 1 was modified by omission of theinitiator ascorbic acid. The resulting smooth-surfaced sphericalparticles were obtained in approximately the same yield and size asobtained in Example 1, but water content was about 4%.

The polymerization process of the present invention produces a drypowder-like particle, eliminating the need for drying, grinding,pulverizing and/or other methods to produce suitable for immediate usein the range of from about 2 to 125,000 microns in size. Moreover, thisinvention utilizes liquid monomer sources to produce a substantially drypolymer particle which does not rely upon a substrate on which toinitiate formation.

The process and apparatus described herein may also be used in thecontinuous preparation of soaps and detergents. The raw materials ofsoap or detergent production are mixed and/or homogenized, and subjectedto treatments, as known to those skilled in the art to prepare a liquidmixture of soap- or detergent-making compounds. The liquid mixture isthen sprayed into the reaction chamber according to the abovedescription. Standard soap and detergent formulations known to thoseskilled in the art may be used to prepare a substantially dry powderedsoap or detergent.

The preparation of a soap or detergent will not require that thetemperature in the feed line of the above-described process andapparatus be reduced. The liquid mixture will however, be subjected topressure in the line and to the spraying and chamber conditionsdescribed above. varying chamber pressure and temperature as describedabove will produce similar results in detergent and soap production asare obtained from monomer polymerization.

The process and apparatus described herein may also be used in thepreparation of several novel polymer products. Foremost, it has beenfound that by altering the operational parameters of the apparatus, notonly is the size and density of the product varied, but the shape andphysical properties of the products are varied as well. Moreparticularly, when the apparatus is operated at lower pressures andhigher temperatures, the water or other solvent in the mixture dropletsis violently released during polymerization. The result is asubstantially dry polymer product which has a puffed out irregular shapenot unlike cooked popcorn. The novelty of this product is believed to bein its irregular puffed shape, its low density, and in the case of thewater absorbing polymers, an enhanced ability to absorb water or urinedue to an increase in the polymer's surface area. Further, a productwith these characteristics is obtained without spray drying oradditional processing as is required in the known processes forobtaining similar products.

Similar known products do not have the same degree of irregularity intheir shape as they are most commonly formed through the mechanicalprocesses of pulverizing and chopping a mass of polymer material. Thus,known irregularly shaped polymers have less surface area and commonlyhave higher densities. Known methods for obtaining irregularly shapedpolymer products are described in U.S. Pat. No. Re. 32,649 entitled“Hydrogel-Forming Polymer Compositions For Use in Absorbent Structures,”reissued to Brandt et al. on Apr. 19, 1988 and U.S. Pat. No. 4,625,001entitled “Method for Continuous Production Of Cross-Linked Polymer”issued to Tsubakimoto et al. on Nov. 25, 1986. These patents areincorporated herein by reference.

An additional novel product is obtained from the process and apparatusdescribed herein in the form of a substantially dry pigmented copolymerproduct for use in powder coating techniques. To obtain this product theprocess further comprises the steps of adding a second monomer to thefirst mixture and the step of adding a pigment to the first mixture. Thehomogenized mixture is then sprayed into the apparatus forming mixturedroplets which polymerize during free fall. The product obtained fromthis process is a substantially dry, pigmented copolymer that isradiation curable for use in powder coating applications; additionalchemical or mechanical processing of the copolymer product is notrequired.

The production of copolymers for use as binders in powder coatings isdescribed in U.S. Pat. No. 5,484,850 entitled “Copolymers CrosslinkableBy A Free Radical Method” issued to Kempter et al. on Jan. 16, 1996.This patents is incorporated herein by reference. As described in theKempter patent, copolymers for use in powder coatings typically requireseveral manufacturing steps in their production. The monomer reagentsare first polymerized into a copolymer. The copolymer is then dispersedor dissolved in an emulsion or solution in order to introduce additivesby various known techniques. Lastly, the copolymer composition is driedin a conventional manner. A substantially dry, colored copolymer isformed from a homogenized mixture of monomers and various additives in asingle step in the process and apparatus described herein.

The term “controlled atmosphere” as used herein describes the internalcondition of the apparatus and the reaction conditions of thepolymerization process. As such, “controlled atmosphere” refers to anatmosphere that is substantially static. Any currents or movement ofgases within the apparatus or in the vicinity of where thepolymerization process is being carried out should be minimized; thereshould be no currents or turbulence present to interfere with the freefall of the polymerizing droplets. While the droplets are generallydescribed as falling through the apparatus, they are more accuratelydescribed as experiencing a free fall, which is preferably influencedonly by the force of gravity.

Those skilled in the art who have the benefit of this disclosure willrecognize that the above examples are set out by way of exemplificationand for the purpose of complying with the requirements of the PatentStatute, and that those examples are not intended to limit the scope ofthe present invention. Likewise, with respect to the apparatus 10, itwill be recognized that changes can be made to the individual structuralelements comprising that apparatus without changing the manner in whichthose elements function to achieve the intended result thereof. All suchchanges are intended to fall within the scope of the following claims.

What is claimed is:
 1. A small particle size, substantially dry, waterabsorbing crosslinked polymer made by the process, comprising the stepsof: preparing a mixture of a monomer, a crosslinker and an initiator inwater; heating the atmosphere of a reaction chamber to between about 70°F. and about 300° F., defined by a vessel; controlling the pressure ofthe atmosphere in the reaction chamber from about 0.1 to about 30.0inches of Hg relative to ambient pressure; spraying the mixture into theheated pressure controlled atmosphere forming free-falling droplets;allowing the droplets to fall through the heated, pressure controlledatmosphere for a period of time sufficient to obtain the desired degreeof polymerization of the monomer and crosslinking of the polymerizedmonomer and to form dry particles of flowable water-absorbing polymerpowder; and removing fallen particles of dry water-absorbing polymerpowder that collect at the bottom of the reaction chamber whilemaintaining the reduced pressure therein and continuously producingpolymer; and wherein a small particle size, substantially dry, waterabsorbing crosslinked polymer is produced, the polymer having asubstantially smooth and spherical shape, a water content of less thanabout 2 percent by weight and a particle size of less than about 50microns.